Heretofore, most fillers for polymeric materials such as metals, graphite powders or carbon blacks possessed a fixed bulk resistivity so that any variations which are required in the electrical properties of the final composition of matter are, of necessity, obtained by varying the concentration of the particular filler which is employed. Thus, when a filler material with a fixed bulk resistivity, such as metal or carbon powder is used, it is difficult to control the desired final resistance if said resistance is in the region of rapidly changing resistance with changing concentration. The electrical resistivity of the final composition of matter for a given filler material will depend on filler concentration inasmuch as at lower filler concentration addition of filler material has very little effect on the resistance of the final composition of matter due to the tendency of polymeric materials to encapsulate the filler, while at higher filler concentrations addition of filler material causes very little change in the filler electrical properties, as is the case at lower filler concentrations, because of effective control of these properties by the bulk resistivity of the filler material. However, at intermediate filler concentrations, addition of filler material causes a rapid change in the final electrical properties because of the onset of filler particle-particle interactions in the polymeric matrix. Heretofore, the control of the electrical properties of the final composition of matter by varying the concentration of filler was a difficult procedure, especially, as hereinbefore set forth, if the final desired resistance was in the region of rapidly changing resistance. The optimum concentration is one in which small changes in filler concentration produce little or no change in resistance of the final product, this usually being achieved at high concentration of filler content. As will hereinafter be set forth in greater detail, by utilizing certain materials as fillers for polymeric materials, it will be possible to obtain final compositions of matter which possess desirable and controlled resistivity.
The prior art has disclosed the use of certain compounds which possess resistive properties. For example, U.S. Pat. No. 2,096,992 discloses the calcination of a combination of tannic acid with a gel-forming combination of materials. The description which is set forth in this patent is that of a finely divided carbon which is uniformly dispersed in a matrix. However, there is no description of the treatment conditions nor is there any description of the finished properties of the product and there is certainly no indication that a product can be produced which possesses variable electrical resistivity. The persistent reference which is made in this patent to carbon suggests that a very high calcination treatment temperature is used, that is, greater than about 1200.degree. C., so that a material approaching graphite in nature is formed. There is nothing in this reference which teaches or suggests the novel composition of matter which is formed by the process of the present invention. Likewise, U.S. Pat. No. 3,576,378 teaches the use of a polymeric material having electrically conductive particles dispersed throughout the polymer. However, this electrically conductive material or conducting powders which are incorporated into the polymeric matrix have a continuous outer surface of metal, examples of the metals which are used to coat any known metal particles including aluminum, nickel, lead, zinc, cadmium, copper, iron, silver or gold. These electrically conductive particles are in no way similar to the carbonaceous pyropolymer composited on a refractory inorganic oxide which comprises the conductive particles of the present invention. Another reference which pertains to electroconductive compositions is U.S. Pat. No. 3,563,916 which utilizes a carrier such as a thermosetting resin containing carbon black. In a similar vein, U.S. Pat. No. 3,836,482 also shows a semiconducting composition containing, as a conductive material therefor, carbon black.
As hereinbefore set forth, the prior art fillers such as graphite, metals or carbon black have a fixed bulk resistivity so that variations in the electrical properties of the final product are obtained only by varying the concentration of the filler employed. The fixed bulk resistivity of carbon blacks is illustrated by reference to publications such as advertising brochures from the Cabot Corporation which lists typical properties of Cabot Formation Process Carbon Blacks which are used for inks, paints, plastics, and papers. The carbon blacks which are listed in various forms do not offer any quantitative data but only list the electrical resistivity as being high, medium or low. Thus, the variations in electrical properties must be dependent upon variations in the filler concentration. As will hereinafter be set forth in greater detail, in contradistinction to the fixed bulk resistivities of the carbon blacks, graphite powders, metals, etc., of the prior art, it will be shown that the bulk resistivity of the particular filler which is employed by the process of the present invention is variable over a wide range. As a result of this, the electrical properties of the final product may be controlled by varying the bulk resistivity of the filler rather than the volume concentration of the filler in the polymeric material, the latter method being the method which must be employed when using the semiconducting material of the prior art. Therefore, it is possible that the optimum filler concentration can be used so that the resistance of the final product is relatively insensitive to variations in the filler concentration as might be obtained in a production process and the resistance of the final product can therefore be precisely controlled by varying the bulk resistivity of the claimed filler.
As will also be hereinafter set forth in greater detail, it will be shown that there are additional inherent advantages to be derived from the use of the semiconducting pyropolymeric inorganic refractory oxide material as the filler in polymeric materials, these advantages resulting from the electrical characteristics of the filler. Among these inherent advantages are (1) controllable resistance change with temperature; (2) low current noise in applications which require quiet contacts to be attached to the final product; (3) excellent resistance stability with respect to temperature cycling; and (4) a high degree of batch to batch reproducibility of the electrical properties of the filler, the latter being in contradistinction to the situations with graphite or carbon where significant variations in reproducibility occur.
This invention relates to novel compositions of matter. More specifically, the invention relates to novel compositions of matter comprising polymeric materials which contain certain semiconducting pyropolymeric inorganic refractory oxide materials as fillers therefor, the final product being useful in the electrical field.
The use of polymeric materials which contain, as fillers therefor, other materials which possess electrical resistance properties are useful in many fields. For example, a polymeric material containing such a filler may be used where controllable resistance changes with temperature are required, as articles or products which possess low current noise, in applications which require quiet contacts to be attached to the final product and in other instances where an excellent resistance stability with respect to temperature cycling is required. By utilizing semiconducting pyropolymeric inorganic refractory oxide materials which possess certain electrical resistances, it is possible to prepare articles of manufacture which may possess conducting properties or which may possess anti-static properties. For example, it can be seen that an anti-static formulation would be extremely suitable in instances in which it is desirable to eliminate electric charges which may be built up by friction such as in synthetic fabrics which become charged by rubbing against another material as in automobile, bus, truck or airplane upholstery. In addition, it may also be most desirable for use in polymeric materials which are utilized in explosive environments such as fuel containers in airplanes, boats, automobiles, trucks, buses, etc., in fuel transfer pipelines such as those which are used to transport gasoline, oil or liquefied petroleum gas, heating gas, etc., in medical operating room surfaces where oxygen may be present with the possibility of sparking taking place, etc.
While the aforementioned discussion of the uses of polymeric materials containing a filler in anti-static situations where the filler possesses an electrical resistance between 50,000 and 10.sup.10 ohms, it is also possible to formulate compositions of matter comprising polymeric materials containing a filler which possess an electrical resistance less than 50,000 ohms, said compositions of matter being used where it is desirable to pass an electrical current through the item. These conducting layers of polymeric materials may be included in wall paneling or ceiling paneling in building construction for heating purposes or may also be used in exterior surfaces whereby de-icing of these surfaces could be accomplished in winter.
It is therefore an object of this invention to provide novel compositions of matter which possess desirable electrical properties.
Another object of this invention is to provide novel compositions of matter which may be used in either conducting or anti-static applications.
In one aspect an embodiment of this invention resides in a composition of matter comprising a polymeric material containing, as a filler therefor, a refractory inorganic oxide having a carbonaceous pyropolymer composited on the surface thereof.
Another embodiment of this invention resides in an electrically conductive sheet which comprises at least one layer of a reinforcing material provided with a polymeric matrix material containing a finely divided semiconductive filler comprising a refractory inorganic oxide having a carbonaceous pyropolymer composited on the surface thereof.
A specific embodiment of this invention resides in a novel composition of matter comprising polypropylene containing, as a filler therefor, a semiconducting pyropolymeric inorganic refractory oxide material possessing a particle size in the range of from about 0.1 microns to about 100 microns which has a bulk resistivity in the range of from about 0.001 ohm-centimeters up to about 10.sup.10 ohm-centimeters, said filler being present in a range of from about 10 to about 95% by weight of the composition.
Another specific embodiment of this invention is found in an electrically conductive sheet which comprises at least one layer of canvas provided with a polymeric matrix material comprising epoxy resin containing a finely divided semiconductive material comprising a refractory inorganic oxide having a carbonaceous pyropolymer composited on the surface thereof.
Other objects and embodiments will be found in the following further detailed description of the present invention.
As hereinbefore set forth the present invention is concerned with novel compositions of matter comprising polymeric materials containing, as a filler therefor, a semiconducting pyropolymeric inorganic refractory oxide material which possesses certain electrical properties including a resistivity of a certain magnitude. The semiconducting pyropolymeric inorganic refractory oxide material comprises a carbonaceous pyropolymer consisting of carbon and hydrogen on a high surface area inorganic oxide support. The magnitude of the resistivity can be varied and may range from about 0.001 up to about 10.sup.10 ohm-centimeters, the particular variation in resistivity being accomplished by varying the procedure in which the semiconducting pyropolymeric inorganic refractory oxide material is prepared. By utilizing this material as a filler for polymeric materials, it is possible to obtain articles which will overcome certain deficiencies which are possessed by articles such as those set forth in the prior art patents which have heretofore been discussed. Among the advantages which are possessed by the semiconducting pyropolymeric inorganic refractory oxide material of the instant application is that the temperature coefficient of resistance of these pyropolymeric materials is controllable over a relatively broad range; that the materials possess a low current noise index variance; that the resistance stability of the semiconducting pyropolymeric inorganic refractory oxide material with respect to temperature cycling can also be controlled to within a relatively narrow range and that the material of the present invention can also be prepared with an excellent batch to batch reproducibility.
The semiconducting pyropolymeric inorganic refractory oxide material which is utilized as a filler for the polymeric material of a type hereinafter set forth in greater detail may be prepared by heating an organic compound in the absence of oxygen and passing the pyrolyzable substance over the refractory oxide material in the vapor phase to form a carbonaceous pyropolymer thereon. The refractory oxide material which may be used as the base may be in any form such as loose or compacted dry powders, cast or calcined sols, heated sols, substrates in the form of flats, cylinders, and spheres, rods, pellets, etc. In the preferred embodiment of the present invention the refractory oxide base will be characterized as having a surface area of from 1 to about 500 square meters per gram. Illustrative examples of the refractory oxides which may be used will include alumina in various forms such as gamma-alumina and silica-alumina. In addition, it is also contemplated that the refractory oxide may be preimpregnated with a catalytic metallic substance such as platinum, platinum and rhenium, platinum and germanium, platinum and tin, platinum and lead, nickel and rhenium, tin, lead, germanium, etc.
Examples of organic substances which may be pyrolyzed to form the carbonaceous pyropolymer on the surface of the aforementioned refractory oxides will include aliphatic hydrocarbons, cycloaliphatic hydrocarbons, aromatic hydrocarbons, aliphatic halogen derivatives, aliphatic oxygen derivatives, aliphatic sulfur derivatives, aliphatic nitrogen derivatives, heterocyclic compounds, organometallic compounds, carbohydrates, etc. Some specific examples of these organic compounds which may be pyrolyzed will include ethane, propane, butane, pentane, ethylene, propylene, 1-butene, 2-butene, 1-pentene, 2-pentene, 1,3-butadiene, isoprene, cyclopentane, cyclohexane, methylcyclopentane, benzene, toluene, the isomeric xylenes, naphthalene, anthracene, chloromethane, bromomethane, chloroethane, bromoethane, chloropropane, bromopropane, isopropane, chlorobutane, bromobutane, isobutane, carbon tetrachloride, chloroform, 1,2-dichloroethane, 1,2-dichloropropane, 1,2-dichlorobutane, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, glycol, glycerol, ethyl ether, isopropyl ether, butyl ether, ethyl mercaptan, n-propyl mercaptan, butyl mercaptan, methyl sulfide, ethyl sulfide, ethyl methyl sulfide, methyl propyl sulfide, dimethyl amine, diethyl amine, ethyl methyl amine, acetamide, propionamide, nitroethane, 1-nitropropane, 1-nitrobutane, acetonitrile, propionitrile, formic acid, acetic acid, oxalic acid, acrylic acid, formaldehyde, acid aldehyde, propionaldehyde, acetone, methyl ethyl ketone, methyl propyl ketone, ethyl propyl ketone, methyl formate, ethyl formate, ethyl acetate, benzyl chloride, phenol, o-cresol, benzyl alcohol, hydroquinone, resorcinol, catechol, anisole, phenetole, benzaldehyde, acetophenone, benzophenone, benzoquinone, benzoic acid, phenyl acetate acid, hydrocynamic acid, furan, furfural, pyran, coumarin, indole, dextrose, sucrose, starch, etc. It is to be understood that the aforementioned compounds are only representative of the class of compounds which may undergo pyropolymerization and that the present invention is not necessarily limited thereto.
As hereinbefore set forth the aforementioned organic compounds are admixed with a carrier gas such as nitrogen or hydrogen, heated and passed over the refractory oxide base. The deposition of the pyropolymer on the surface of the base is effected at relatively high temperatures ranging from about 400.degree. to about 800.degree. C. and preferably in a range of from about 600.degree. to about 750.degree. C. It is possible to govern the electrical properties of the semiconducting pyropolymeric inorganic refractory oxide material by regulating the temperature and the residence time during which the refractory oxide base is subjected to the treatment with the organic pyrolyzable substance. The thus prepared semiconducting pyropolymeric inorganic refractory oxide material when recovered will possess a resistivity in the range of from about 10.sup.-2 to about 10.sup.8 ohm-centimeters. However, if so desired, the semiconducting pyropolymeric inorganic refractory oxide material may also be subjected to additional exposure to elevated temperatures ranging from about 500.degree. to about 1200.degree. C. in an inert atmosphere and in the absence of additional pyrolyzable materials for various periods of time, said treatment resulting in the reduction of the electrical resistivity of the lowest resistivity powders by as much as six orders of magnitude. Another method of preparing the desired material is by impregnating the inorganic refractory oxide material with an aqueous solution of a carbohydrate such as dextrose, sucrose, starch, etc. Following the impregnation, the oxide material is dried and thereafter is pyrolyzed at a temperature in the range hereinbefore set forth for a predetermined period of time in order to obtain a semiconducting pyropolymeric inorganic refractory oxide material which possesses a resistivity within the range hereinbefore set forth. While the above material describes two specific methods of preparing a semiconducting pyropolymeric inorganic refractory oxide material, it is to be understood that we do not wish to be limited to these methods of preparing said material and that any suitable method in which at least a mono-layer of a carbonaceous material is formed on the surface of a refractory oxide material may also be utilized to form the desired filler.
The semiconducting pyropolymeric inorganic refractory oxide material which is prepared according to one of the processes hereinbefore set forth in the preceding paragraphs and which will possess resistivities in the range of from about 0.001 to about 10.sup.10 ohm-centimeters may be admixed with virtually any polymeric material which can act as a host matrix for the filler. Some specific examples of these polymers which may be both thermosetting or thermoplastic by nature will include polyolefins such as polyethylene and polyethylene copolymers, polypropylene and polypropylene copolymers, polystyrene and copolymers, polyvinylacetate, polyvinyl chloride, vinylacetate-vinyl chloride copolymers, polyvinylidene chloride and copolymers, etc., polyesters, polyurethane, polyphenyl ethers, styrenated polyphenyl ethers, polycarbonates, polyamides, polyimides, polyoxymethylenes, polyalkylene oxides such as polyethylene oxide, polyacrylates, polymetacrylates and their copolymers with styrene, butadiene, acrylonitrile, etc., epoxy resins, acrylonitrile-butadiene-styrene formulations (commonly known as ABS), polybutylene and acrylic-ester-modified-styrene-acrylonitrile (ASA), alkyd resins, allyl resins, amino resins, phenolic resins, urea resins, melamine resins, cellulose acetate, cellulose acetate butyrate, cellulose nitrate, cellulose propionate, cellulose triacetate, chlorinated polyethers, chlorinated polyethylene, ethyl cellulose, furan resins, synthetic fibers such as the Nylons, Dacrons, Rayons, terylenes, etc. In addition other matrices which may be used as a host for the semiconducting pyropolymeric inorganic refractory oxide material filler may comprise laminates which are formed by treating a reinforcing material such as canvas, asbestos, glass cloth, cardboard, paper, etc., with a monomer or polymer containing the semiconducting pyropolymeric inorganic refractory oxide material fillers and thereafter forming the desired laminate by conventional means. For example, canvas may be impregnated with such a thermosetting phenolic resin and the resulting composition of matter heated for a predetermined period at an elevated temperature of about 250.degree. to 350.degree. C. to form the desired product. Likewise, asbestos may be treated with vinyl chloride and vinyl acetate dissolved in a solvent which is thereafter allowed to evaporate thus forming the laminate. It is also possible to treat a reinforcing material such as canvas with a mixture of self-catalyzing epoxy resins and allowing the results to set up at room temperature. It is to be understood that the aforementioned polymeric materials are only representative of the class of compounds which may be composited with the fillers to form the novel compositions of matter of the present invention, and that said present invention is not necessarily limited thereto.
The compositions of matter of the present invention may be composited by any method known in the art. The filler material which comprises the semiconducting pyropolymeric inorganic refractory oxide material which possesses the desired resistance is comminuted by milling the material to form particles which possess the desired size, that is, less than 100 microns in size and preferably to form particles less than 1 micron. These particle sizes can be obtained by wet milling the filler material in a volatile solvent medium by means of a roll mill, colloidal mill or ball mill and thereafter flashing off or evaporating the solvent to obtain the dried powder. Examples of suitable solvents which may be employed in the wet milling process will include alcohols, ethers and ketones, etc., such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, acetone, methyl isobutyl ketone, methyl ether, ethyl ether, etc., the evaporation or flashing off step being effected at temperatures ranging up to about 100.degree. C. or more. Following this, the powdered filler in the necessary particle size may then be admixed with a polymeric material of the type hereinbefore set forth, again in any suitable manner, the semiconducting pyropolymeric inorganic refractory oxide filler material usually being present in an amount in the range of from about 95 to about 10% by weight of said filler per weight of the finished composition of matter. One such type of mixing which will permit a thorough and uniform distribution of the filler throughout the body of the polymer is to admix said filler with the polymer in a roll mill, said process being especially effective when the polymer is also in dry form. If so desired, the mixture may be further milled to adjust the particle size of the filler. Following this, a solvent of the type hereinbefore set forth may also be added and the resulting mixture stirred until a uniform consistency has been reached. Thereafter the plastic may then be utilized for the final purpose such as a coating, being poured into a mold whereby the desired form or shape is obtained. When the prepolymeric material is in liquid form such as the case of some epoxy resins, the filler is poured into the liquid and the resulting mixture stirred in order that a uniform distribution of the filler may be obtained throughout the body of the liquid. The resulting composition of matter is then poured into molds or used as a coating which is set by thermal means in the case of thermosetting resins or otherwise cured.